Laterite nickel and cobalt ore deposits generally contain oxidic type ores, limonites, and silicate type ores, saprolites, in the same deposits. The higher nickel content saprolites tend to be treated by a pyrometallurgical process involving roasting and electrical smelting techniques to produce ferro nickel. The power requirements and high iron to nickel ore ratio for the lower nickel content limonite and limonite/saprolite blends make this processing route too expensive, and these ores are normally commercially treated by a combination of pyrometallurgical and hydrometallurgical processes, such as the High Pressure Acid Leach (HPAL) process or the Caron reduction roast—ammonium carbonate leach process.
As alternatives to HPAL, which treats limonite or low magnesium laterites only and uses expensive high pressure equipment, atmospheric pressure agitation acid leach processes, and processes combining HPAL for the limonite fraction of an ore followed by atmospheric acid leaching of the saprolite fraction have been disclosed. In order to reduce the size of leaching reactors, high grade limonite and saprolite are preferred for these processes. This leads to rejecting the low grade ore as waste.
The exploitation of many of the lower nickel content ores by the above processes generally requires whole ore processing as there is no effective method to beneficiate the ore. This has the disadvantage that the mineralogical fractions of the ore which may contain lower metal values effectively dilute the total treated ore quality and increase recovery costs.
Even where the laterite ore is amenable to some form of beneficiation, where the upgraded ore is processed by one of the previously discussed methods, the reject fraction containing low nickel and cobalt grades is normally discarded as uneconomic to process by the above methods, thus losing the value of the nickel and cobalt contained in the rejects.
Heap leaching is a conventional method of economically extracting metals from low grade ores and has been successfully used to recover materials such as copper, gold, uranium and silver. Generally it involves piling raw ore directly from ore deposits into heaps that vary in height. The leaching solution is introduced onto the top of the heap to percolate down through the heap. The effluent liquor is drained from the base of the heap and passes to a processing plant where the metal values are recovered.
One problem hindering the heap leaching of nickel and cobalt containing laterite ores is the substantial clay component of such ores. The type of clay content is dependent on the parent rock and the physico chemical environment of the clay formation, but most clays have a detrimental effect on the percolation of the leach solution through the ore.
It has been reported that when laterite is piled dry, the leach solution percolation was poor to impossible. Because of the poor permeability, a low irrigation rate is necessary to allow the solution to leach the nickel and cobalt, thus requiring a leach time that is uneconomical.
U.S. Pat. No. 5,571,308 (BHP Minerals International, Inc) describes a process for heap leaching of high magnesium containing laterite ore such as saprolite. The patent points out that the clay type saprolite exhibits poor permeability, and as a solution to this, pelletisation of the ore is necessary to ensure distribution of the leach solution through the heap.
U.S. Pat. No. 6,312,500 (BHP Minerals International, Inc) also describes a process for heap leaching of laterites to recover nickel, which is particularly effective for ores that have a significant clay component (greater than 10% by weight). The process includes sizing of the ore where necessary, forming pellets by contacting the ore with a lixivant, and agglomerating. The pellets are formed into a heap and leached with sulphuric acid to extract the metal values.
Both the above patents identify the need to pelletise the whole ore feed to obtain the permeability of the heap necessary for successful heap leaching.
The above discussion of documents, articles and the like is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date.
The present invention aims to overcome or at least alleviate one or more of the difficulties associated with the prior art.